Water Companies employ filters to remove the fine particles of dirt that remain afloat in the water after it has been clarified.
Historically filters were made of sand and gravel, the gravel being used merely to prevent the sand from falling out of the filter. These filters are very effective at removing particulate matter from a liquid such as water. However, in waters where algae are abundant, the water can develop a foul taste and a slight colour. Sand and gravel filters will not remove the agents which cause these problems. Hence, nowadays Water Companies employ Carbon filters which use “Granular Activated Carbon” instead of sand. The granular form means that the carbon will remove particulate matter just as effectively as sand, however the added advantage is that it will also chemically remove the agents responsible for foul tastes and colouration.
In class you may wish to test the effectiveness of a series of filter materials at separating solids from liquids, e.g. coffee filter papers, tea bags, paper towels, cotton wool, sand, gravel, different fabrics etc.
To make the filters follow the instructions below:
1: Take a 2 litre plastic pop bottle and cut it in two about halfway up the bottle
2: Turn the top half of the bottle upside down and insert it into the bottom half, so that the top of the bottle forms a funnel that will drain into the bottom half.
3: Place the materials to be tested in the upturned top half either singly or in layers.
To make the mixture of liquid and solid that is to be separated, use substances such as sand, dirt, iron filings, etc. You may choose to use soluble materials such as sugar, salt or food colourings to show how sand filters are unable to remove non-particulate or non-solid substances.
The best filters usually consist of two or three layers, the top being made of sand, with a gravel bed to hold the sand in the filter, and sometimes a small cloth plug put in the neck of the bottle.
Be sure to use pre-washed sand and gravel otherwise the filters will make the water dirtier than it was before you started.
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